On the reverse link (uplink) of a CDMA (code division multiple access) wireless system based on the TIA standard IS-95 or ANSI standard STD-J-008, the coding scheme involves, among other things (like convolutional coding), the use of Walsh coding, as known in the art. In this form of Walsh coding, groups of data, each group comprising 6 bits, are mapped into one of 64 Walsh codewords before transmission. In other words, the value of the 6 bits provide an index into one of the 64 Walsh codewords.
In general, the demodulator for this scheme (located in the base-station receiver) employs a RAKE receiver with multiple fingers, as known in the art, where each finger is capable of locking onto and demodulating a received multi-path signal. Each such finger comprises a bank of 64 Walsh correlators where each Walsh correlator is tuned to a different one of the 64 possible Walsh code words and is identified by a corresponding 6 bit index value as used in the transmitter. The output signals of the corresponding Walsh correlators in each active finger (i.e., those that are locked on to a multipath) are summed to obtain 64 output signals--one for each Walsh index. For simplicity, the remainder of this description of the prior art and the inventive concept (described further below) is in the context of a single finger RAKE receiver with one bank of Walsh correlators.
As part of the demodulation process, a received signal, after undergoing despreading, is fed to a bank of Walsh correlators for correlation with each Walsh codeword. The output signals of the Walsh correlators are fed to a soft decision metric generator along with the corresponding Walsh indices. The soft decision metric generator provides a decision metric, D.sub.k, for each bit of a selected Walsh index (k=1, 2, . . . , 6) for subsequent use by a Viterbi decoder. As such, the soft decision metric generator is a critical component of the base-station receiver and its performance can significantly affect the signal-to-noise ratio (SNR) requirement for the reverse link.
One method for soft decision metric generation is known as the conventional method (e.g., see "Digital Communications," Proakis, J., 2nd Ed., McGraw-Hill, New York, 1989). In particular, let Z.sub.0, Z.sub.1 . . . , Z.sub.63 . . . , denote the corresponding energy levels of the 64 correlator output signals passed to the decision metric generator. In the conventional method, the soft decision metric generator makes a hard decision about the transmitted Walsh code by selecting the Walsh index associated with the correlator output signal having the highest energy level. The soft decision metric generator then passes a decision metric to a Viterbi decoder by weighting the corresponding values of the selected index bits as a function of this highest energy level. To see how the conventional method works, let Z.sub.max be the energy level of the selected (i.e., highest energy) correlator output signal, and let I.sub.max ={I.sub.max (1), I.sub.max (2), I.sub.max (6)} be the corresponding Walsh index, where the 6-bit vector I.sub.max is a binary representation of the selected Walsh index. The resulting decision metric for the k.sup.th bit (k=1, 2, . . . , 6) covered by this Walsh codeword is give by:
D.sub.k =+Z.sub.max, if I.sub.max (k)=1; and PA1 D.sub.k =-7-max, if I.sub.max (k)=0. PA1 D.sub.k =Max Z.sub.I -Max Z.sub.I.
Note, that in this algorithm all bits (code symbols) of the selected index are weighted (+/-) by the same energy level, Z.sub.max. While the conventional method is relatively simple to implement (in terms of the required complexity of ASIC (application-specific integrated circuit) implementation), its performance falls considerably short of the near-optimal method (e.g., see "CDMA--Principles of Spread Spectrum Communication," Viterbi, A. J., Addison Wesley, Reading Mass., 1995).
In the near-optimal method, and using the same notation as before, assume that Z.sub.0, Z.sub.1, . . . Z.sub.63, are corresponding energy levels of the 64 correlator output signals for a received Walsh codeword. The decision metric for the k.sup.th bit (k=1, 2, . . . , 6) is the energy difference between the largest correlator output signal whose index has a 1 in the k.sup.th position and the largest correlator output signal whose index has a 0 in the k.sup.th position. That is,
I:I(k)=1 I:I(k)=0
The near-optimal method is known to yield considerably superior performance (in terms of the required SNR) at the expense of a significant increase in receiver complexity.